The driving seat of a motor vehicle is generally adjustable so that the driver can adapt the height of the seat to suit his own size with a view to optimal driving comfort.
To do this, a vehicle seat generally comprises a seat frame which can be adjusted relative to a base frame, generally anchored to the floor of the vehicle.
The height of the seat frame with respect to the base frame is adjusted using an actuator mounted between the said base frame and the said seat frame.
The actuator may, but need not necessarily, be a gas spring actuating cylinder.
This actuator is supplied by a source of pressure and is associated with a control valve for adjusting the extension or retraction of the actuator and therefore the height of the seat frame in relation to the base frame.
Furthermore, height adjustment systems may also be equipped with a function of memorizing and maintaining the height, particularly as a function of the weight of the driver.
A system such as this is known and described in document US 2009/0026816.
Such an adjusting system therefore comprises:                a raising or “UP” function for raising the seat and which is manifested by the pressurizing of the actuating cylinder,        a lowering or “DOWN” function for lowering the seat and which is manifested by the exhausting of the actuating cylinder,        a regulating or “AUTOLEVEL” function which is able to compensate for variations in height of the seat about a predefined position stored in memory. This function is manifested by the adjusting of the pressure in the gas spring actuating cylinder.        
In general, the UP and DOWN functions have the effect of resetting the memory of the AUTOLEVEL function, the reference height being altered.
Furthermore, a system like the one described in document US 2009/0026816 entails a large number of components, and in particular, at least two sets of valves.
There is thus a need for a more simple and effective system that displays additional functionalities.